The present invention relates generally to apparatus for sputter deposition of optical coatings, and more particularly to continuous in-line sputtering apparatus for controlling the uniformity of the deposited layers.
Reactive sputtering is the process most often used for large area commercial coating applications. Typical applications are thermal control coatings for architectural and automobile glazings. In this process, the articles to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. Such an apparatus is referred to as an in-line system or a glass coater. Inside the vacuum chambers, a gas discharge, the sputtering discharge, is maintained. The sputtering discharge pressure is held between about 1 to 5 millitorr, by constantly admitting a mixture of an inert gas, such as argon, with a small proportion of a reactive gas, for example oxygen, for the formation of oxides. Each chamber contains one or more cathodes held at a negative potential in the range from about -200 to -1000 volts. The cathodes may be in the form of elongated rectangles, the length of which spans the width of the chambers. The cathodes are typically 0.10 to 0.30 meters wide and a meter or greater in length. The cathodes are surfaced with a metal with which the reactive gas will form the appropriate compound. This metal surface is frequently referred to as the sputtering target. The cathodes include a magnet array which concentrates the sputtering activity in a narrow region on the target called the sputtering zone. This type of cathode is known as a magnetron cathode.
Ions from the discharge are accelerated into the cathode and dislodge, or sputter off, atoms of the target metal which are deposited on the substrate passing beneath them. The metal atoms react on the substrate with the reactive gas in the discharge to form a thin film of the desired coating material.
The cathodes of an in-line sputtering machine are usually very long compared with their width. It is generally assumed that a substrate, passed beneath the cathode so that it's surface plane is parallel to the sputtering surface of the cathode, will receive a film of equal thickness across its entire width. It is understood that to ensure a uniformly thick film, care must be taken to make sure that the magnetron's magnetic field is constant along the surface of the cathode. It is also understood that some loss of uniformity is inevitable at the extreme ends of the cathode where the magnet array is terminated. This loss of uniformity is referred to as the "end effect". It is generally accepted that articles requiring a more or less uniform film should not be wider than the cathode length minus twice the "end effect" length. In practice, uniformity in general may be affected by other factors, such as the gas flow distribution and the sputtering gas discharge potential around the cathode.
Several different approaches to controlling coating uniformity have been taken. These include specially designed gas distribution systems surrounding the sputtering cathodes. On planar magnetron sputtering cathodes, fixed masks or shields have been used at the edge of the cathode's sputtering zone to shape the flux of sputtered material from the cathode. The problem common to these approaches is that they are static, that is, adjustments are only possible by stopping the process and opening the machine to make the adjustment.
Although it is generally believed that sputtering conditions in an in-line coater may be held stable over several days of operation, changes, particularly in the gas flow and the discharge potential, in fact, occur as the machine is operated. Coatings such as low emissivity (low-E) coatings may tolerate film thickness variations of about plus or minus five percent. Changes in the sputtering conditions necessary to create a plus or minus five percent change in film thickness may not be detectable during a normal operating period of, for example, one or two days.
High precision optical coatings, such as multilayer antireflection coatings, will not tolerate layer thickness variations much greater than plus or minus one percent. The variations are detectable as changes in the reflection color of the coated substrate. Changes in sputtering conditions producing one percent variations in coating thickness may be detectable in the first one or two hours of machine operation. The cost effectiveness of in-line coating systems is based on the ability to operate a process uninterrupted for periods of several days. It is not cost effective if a machine has to be opened every two hours to adjust a static mask or gas flow nozzles to restore film thickness uniformity.
Accordingly, it is an object of the present invention to provide a system whereby coating uniformity may be adjusted during the deposition process.
It is a further object of the present invention to provide a system whereby coating thickness profiles may be varied to accommodate different substrate configurations.